The invention relates to detection systems and methods and, in particular, to detection systems and methods which are used to detect objects or people moving in the vicinity of a predetermined path or line defined by a distributed antenna, for example an open transmission line. The invention is especially applicable to the detection of intruders or escapees.
Known such detection systems use at least one open transmission line, usually a leaky cable, as a distributed receiving antenna to receive a radio frequency signal transmitted from an associated antenna; or as a transmitting antenna to transmit signals for reception by a separate antenna. An intruder or escapee, or other object, moving in the vicinity of the leaky cable causes a perturbation in the coupling of continuous wave RF energy into or from the leaky cable. Detection of the perturbation indicates an intrusion or escape attempt. It will be appreciated that there is technically no distinction between an intruder traversing the path to enter a protected zone and an escapee traversing the path to leave a protected zone. For convenience, therefore, in this specification, the term xe2x80x9cintruderxe2x80x9d will be used to cover both.
It is desirable to determine, at least approximately, the location of the intruder along the length of the cable. U.S. Pat. No. 4,994,789 (Harman) issued Feb. 19, 1991 discloses a detection system in which several detection zones are provided by interposing phase-shifting modulators at intervals along the leaky cable. Each modulator can be shunted by a switch. A signal processor analyzes the signal received from the cable while the switch is operated so as to shunt the modulator or connect it in series with the cable sections, thereby allowing determination of the section in which the intrusion occurred. When such a system uses only two zones, it may be relatively economical. However, when such a system is expanded to many zones, the interdependence of the modulators, the complexities of switching them, and intricacies of signal analysis prohibitively increase cost and reduce reliability.
U.S. Pat. No. 4,887,069 (Maki) issued Dec. 12, 1989 discloses a detection system which uses two coaxial cables, one of them a leaky cable, extending along a perimeter of a protection zone, one coupled to a transmitter and the other to a receiver. The cables are subdivided into sections which are interconnected by oscillators and switches allowing selection of one section at a time. If a section has not been selected, the RF signal passes along its inner conductor. When a section is selected, the RF signal is switched to propagate as an external wave along the outer sheath of the cable section. Maki also discloses a system in which both of the coaxial cables are leaky cables, with zones provided by serialized switching, each zone being powered from a switched local oscillator. In either case, signal perturbations caused by an intruder are transmitted through the intervening sections to a receiver located at one end of the cable. The oscillators and switches increase complexity and reduce reliability.
In either of these known systems, the radio frequency energy from each cable section must pass through any preceding sections. Consequently, failure of an oscillator/switch, or modulator, as the case may be, especially near the receiver end of the cable, may compromise the system or even render it inoperative.
The present invention seeks to provide a detection apparatus and method with a plurality of detection zones, while eliminating or at least mitigating the aforementioned disadvantages of known detection systems.
According to one aspect of the present invention, a detection system for detecting intruders moving in the vicinity of a defined path comprises a distributed antenna, for example an open transmission line, extending along the path and an array of discrete antennas extending alongside the distributed antenna and within a predetermined distance therefrom, the antennas being spaced apart from each other and the distributed antenna and defining a plurality of detection zones, a radio frequency transmitter connected to one of the distributed antenna and the array of discrete antennas, a complementary receiver connected to the other of the distributed antenna and the array of discrete antennas, and control means for controlling the transmitter, receiver and array of antennas to exchange radio frequency energy several times via the distributed antenna and selected ones of the array of antennas and to analyze the energy received from each selected one of the array of antennas so as to detect perturbations caused by an intruder moving adjacent said path and adjacent that particular antenna.
The control means may comprise switching means for selecting each one of the array antennas individually for such energy exchange.
The control means may select the antennas in turn in such a sequence that, if the energy from a particular antenna when previously selected within a prescribed time period showed a perturbation, that antenna would be selected more frequently than those antennas which had not shown such a perturbation within said time period.
Preferably, the array of antennas are each connected to a respective one of a plurality of taps distributed along a transmission line extending alongside the distributed antenna The control means then may comprise a plurality of switching devices for connecting respective ones of the antennas to the transmission line and switch control means for controlling operation of the switching devices to select the antennas individually.
The switch control means may comprise a means for transmitting antenna addresses selectively onto the transmission line and each switching device then may comprise an address decoder for detecting the address of the associated antenna and an RF switch operable by the decoder to connect the antenna to the transmission line.
According to a second aspect of the invention, there is provided a method of operating an intrusion detection system having a distributed antenna extending along a path to be monitored and an array of discrete antennas extending alongside the distributed antenna and within a predetermined distance therefrom, the method comprising the steps of:
exchanging radio frequency energy several times between the distributed antenna and each of the array of discrete antennas, in turn, and analyzing energy received by each selected antenna to detect perturbations in the received radio frequency energy indicative of an intruder moving near the path and adjacent that antenna.
Preferably, the array antennas are selected in turn in such a sequence that, if the energy from a particular array antenna when previously selected within a prescribed time period showed a perturbation, that array antenna will be selected more frequently than those array antennas which had not shown such a perturbation within said time period.
Thus, the selection sequence may comprise alternate selection of perturbation-indicating antennas with non-perturbation indicating antennas.
Alternatively, the selection sequence may select all of the perturbation-indicating antennas after each selection of a non-perturbation-indicating antenna.
In addition to selecting a perturbation-indicating antenna for which a perturbation was previously recorded, the selection sequence may also be arranged to select one or each of the antennas adjacent thereto.
In embodiments of either aspect of the invention, the array antennas may be fastened to a fence, trees, a wall or a roof and the distributed antenna may be a leaky cable or the like buried nearby.
Preferably, each array antenna is a xe2x80x9csmallxe2x80x9d antenna, i.e. no dimension of the antenna exceeds one tenth of the wavelength of the operating signal and the antenna has substantially no directivity.
When such a small antenna is mounted upon a fence or other generally unstable support, it may be susceptible to movement of the fence or other support. A third aspect of the invention addresses this problem by means of a series resistance of between 150 ohms and 300 ohms between the tap and the antenna element, preferably about 220 ohms.
According to a third aspect of the invention, a small radio frequency antenna suitable for mounting upon an unstable support, such as a wire fence, comprises an insulating board having first and second opposite surfaces, a conductive layer on the first surface, means for attaching the antenna to the support with the second surface opposed to the support, and a resistance means for connecting the conductive layer to a feedline, the resistance having a value between about 150 ohms and about 300 ohms.